Provide operator==() for QIcon

QIcon is a value type with copy and move semantics. However, it doesn't implement operator==, which blocks it from being usable with the new QProperty framework.

Implementing an operator== that does a pixel comparison is probably overkill, also because the icon engine might do whatever it wants when generating the actual pixmaps.

Comparing two QIcon instances as equal when they are copies of each other would be trivial and cheap, and generally delivers on the expectation that

QIcon two = one;
Q_ASSERT(two == one);